Diabetes mellitus (hereinafter, diabetes) currently afflicts at least 200 million people worldwide. The two main sub-types of diabetes include types 1 and 2. Type 1 diabetes accounts for about 10% of the 200 million afflicted with diabetes. Type 1 diabetes is caused by autoimmune destruction of insulin-secreting β-cells in the pancreatic islets of Langerhans. Type 2 diabetes accounts for the remaining 90% of individuals afflicted, and the prevalence is increasing. Type 2 diabetes is often, but not always, associated with obesity, and although previously termed late-onset or adult-onset diabetes, is now becoming increasingly more prevalent in younger individuals. Type 2 diabetes is caused by a combination of insulin resistance and inadequate insulin secretion.
The Physiological Role of Insulin
In a non-stressed normal individual, the basal glucose level will tend to remain the same from day to day because of an intrinsic feedback loop. Any tendency for the plasma glucose concentration to increase is counterbalanced by an increase in insulin secretion and a suppression of glucagon secretion, which regulate hepatic glucose production (gluconeogenesis and release from glycogen stores) and tissue glucose uptake to keep the plasma glucose concentration constant. If the individual gains weight or becomes insulin resistant for any other reason, blood glucose levels will increase, resulting in increased insulin secretion to compensate for the insulin resistance. Therefore the glucose and insulin levels are modulated to minimize changes in these concentrations while relatively normal production and utilization of glucose are maintained.
Five different phases of insulin secretion have been identified: (1) basal insulin secretion wherein insulin is released in the postabsorptive state; (2) the cephalic phase wherein insulin secretion is triggered by the sight, smell and taste of food, before any nutrient is absorbed by the gut, mediated by pancreatic innervation; (3) early-phase insulin secretion wherein an initial burst of insulin is released within the first 5-10 minutes after the β-cell is exposed to a rapid increase in glucose, or other secretagogues; (4) second-phase insulin secretion wherein the insulin levels rise more gradually and are related to the degree and duration of the stimulus; and (5) a third-phase of insulin secretion that has only been described in vitro. During these stages, insulin is secreted, like many other hormones, in a pulsatile fashion, resulting in oscillatory concentrations in the blood. Oscillations include rapid pulses (occurring every 8-15 minutes) superimposed on slower oscillations (occurring every 80-120 minutes) that are related to fluctuations in blood glucose concentration.
Insulin secretion can be induced by other energetic substrates besides glucose (particularly amino acids) as well as by hormones and drugs. Of note is that the insulin response observed after food ingestion cannot be accounted for solely by the increase in blood glucose levels, but also depends on other factors such as the presence of free fatty acids and other secretagogues in the meal, the neurally activated cephalic phase and gastrointestinal hormones.
When an individual is given an intravenous glucose challenge, a biphasic insulin response is seen which includes a rapid increase with a peak, an interpeak nadir and a subsequent slower increasing phase. This biphasic response is only seen when glucose concentration increases rapidly, such as after a glucose bolus or glucose infusion. A slower increase in glucose administration, what is seen under physiologic conditions, induces a more gradually increasing insulin secretion without the well-defined biphasic response seen in response to bolus infusion of glucose.
Modeling of early-phase insulin responses under normal physiologic conditions has demonstrated that, after a meal, glucose concentration increases more gradually (Cmax reached in approximately 20 minutes) than seen with intravenous bolus injections of glucose (Cmax reached in approximately 3-10 minutes).
Healthy pancreatic β-cells generate an early response to a meal-like glucose exposure that rapidly elevates serum insulin both in the portal circulation and in the periphery. Conversely, defective β-cells, which have an impaired early-phase insulin response, generate a sluggish response to the meal-like glucose exposure.
Increasingly, evidence indicates that an early relatively rapid insulin response following glucose ingestion plays a critical role in the maintenance of postprandial glucose homeostasis. An early surge in insulin concentration can limit initial glucose excursions, mainly through the inhibition of endogenous glucose production. Therefore the induction of a rapid insulin response in a diabetic individual is expected to produce improved blood glucose homeostasis.
In a normal individual, a meal induces the secretion of a burst of insulin, generating a relatively rapid spike in serum insulin concentration that then decays relatively quickly (see FIG. 1). This early-phase insulin response is responsible for the shut-off, or reduction, of glucose release from the liver. Homeostatic mechanisms then match insulin secretion (and serum insulin levels) to the glucose load. This is observed as a slow decay of modestly elevated serum insulin levels back to baseline and is second-phase kinetics.
Diabetes
A central characteristic of diabetes is impaired β-cell function. One abnormality that occurs early in the disease progression in both type 1 and 2 diabetes is the loss of eating-induced rapid insulin response. Consequently, the liver continues to produce glucose, which adds to the glucose that is ingested and absorbed from the basic components of a meal.
Type 2 diabetics typically exhibit a delayed response to increases in blood glucose levels. While normal individuals usually begin to release insulin within 2-3 minutes following the consumption of food, type 2 diabetics may not secrete endogenous insulin until blood glucose begins to rise, and then with second-phase kinetics, that is a slow rise to an extended plateau in concentration. As a result, endogenous glucose production is not shut off and continues after consumption and the patient experiences hyperglycemia (elevated blood glucose levels). Another characteristic of type 2 diabetes is impaired insulin action, termed insulin resistance. Insulin resistance manifests itself as both a reduced maximal glucose elimination rate (GERmax) and an increased insulin concentration required to attain GERmax. Thus, to handle a given glucose load more insulin is required and that increased insulin concentration must be maintained for a longer period of time. Consequently, the diabetic patient is also exposed to elevated glucose concentrations for prolonged periods of time, which further exacerbates insulin resistance. Additionally, prolonged elevated blood glucose levels are themselves toxic to β cells.
Type 1 diabetes occurs as a result of the destruction of the insulin-producing cells of the pancreas (β-cells) by the body's own immune system. This ultimately results in a complete insulin hormone deficiency. Type 2 diabetes arises from different and less well understood circumstances. The early loss of early phase insulin release, and consequent continual glucose release, contributes to elevated glucose concentrations. High glucose levels promote insulin resistance, and insulin resistance generates prolonged elevations of serum glucose concentration. This situation can lead to a self-amplifying cycle in which ever greater concentrations of insulin are less effective at controlling blood glucose levels. Moreover, as noted above, elevated glucose levels are toxic to the β-cells, reducing the number of functional β-cells. Genetic defects impairing the growth or maintenance of the microvasculature nourishing the islets can also play a role in their deterioration (Glee, S. M., et al. Nature Genetics 38: 688-693, 2006). Eventually, the pancreas becomes overwhelmed, and individuals progress to develop insulin deficiency similar to people with type 1 diabetes.
Therapy
Insulin therapy is the standard treatment for type 1 diabetes. While incipient type 2 diabetes can be treated with diet and exercise, most early stage type 2 diabetics are currently treated with oral antidiabetic agents, but with limited success. Patients generally transition to insulin therapy as the disease progresses. These treatments, however, do not represent a cure.
In a typical progression the first oral antidiabetic agent used is metformin, a supressor of hepatic glucose output. Use of metformin is not associated with weight gain or hypoglycemia. If metformin treatment is insufficient to control hyperglycemia, an insulin secretagogue, most typically a sulfonylurea, can be added to the treatment regimen. Secretagogues raise the basal level of insulin in order to lower average blood glucose levels. Use of sulphonylureas is associated with weight gain and can lead to hypoglycemia, although severe hypoglycemia is infrequent. If this combination of two oral antidiabetic agents is inadequate to control hyperglycemia either a third oral agent, such as a glitazone, or a long-acting, basal insulin can be added to the regimen. As the disease progresses, insulin therapy can be intensified by the addition of intermediate and short (rapid) acting insulin preparations administered in association with at least some of the day's meals.
Current insulin therapy modalities can supplement or replace endogenously-produced insulin to provide basal and second-phase-like profiles but do not mimic early-phase kinetics (see FIG. 2). Additionally, conventional insulin therapy often involves only one or two daily injections of insulin. However, more intensive therapy such as three or more administrations a day, providing better control of blood glucose levels, are clearly beneficial (see for example Nathan, D. M., et al., N Engl J Med 353: 2643-53, 2005), but many patients are reluctant to accept the additional injections. Use of these conventional insulin preparations is associated with weight gain and a significant risk of hypoglycemia including severe, life-threatening hypoglycemic events.
Until recently, subcutaneous (SC) injection has been the only route of delivering insulin for self-administration by patients commercially available. However, SC insulin administration does not lead to optimal pharmacodynamics for the administered insulin. Absorption into the blood (even with rapid acting insulin analogues) does not mimic the prandial physiologic insulin secretion pattern of a rapid spike in serum insulin concentration. Subcutaneous injections are also rarely ideal in providing insulin to type 2 diabetics and may actually worsen insulin action because of delayed, variable and slow rate of absorption into the bloodstream. It has been shown, however, that if insulin is administered intravenously with a meal, early stage type 2 diabetics experience the shutdown of hepatic glucose release and exhibit increased physiologic glucose control. In addition their free fatty acids levels fall at a faster rate than without insulin therapy. While possibly effective in treating type 2 diabetes, intravenous administration of insulin is not a reasonable solution, as it is not safe or feasible for patients to intravenously administer insulin at every meal.
For a short period of time there was an inhalable insulin, EXUBERA® (Pfizer), which was marketed for the treatment of diabetes. This insulin preparation had a pharmacokinetic profile similar to the injectable rapid acting analogues and was used as a substitute for short acting insulin in the standard treatment paradigm. While this insulin preparation did allow patients using short acting insulins to avoid injections, it offered no other notable advantage which contributed to its commercial failure. Moreover, because its kinetic profile was so similar to subcutaneously administered regular and rapid-acting insulins, that after accounting for differences in bioavailability, its dosing and modes of administration could generally follow that of those subcutaneous insulins.
Though not yet commercially available, an ultrarapid acting insulin, insulin-fumaryl diketopiperazine (FDKP) has been under development. Growing experience with the use of this insulin formulation in human studies is showing that its unique kinetic profile can accommodate different dosing schemes and modes of administration as its use is applied to various situations and patient populations in order to achieve improved glycemic control. Such methods are the object of the present disclosure.